Synchronous buck DC/DC converter to perform an improved switching operation by adjusting variable resistor

ABSTRACT

A synchronous buck DC/DC converter to perform an improved switching operation by adjusting a variable resistor is provided. The synchronous buck DC/DC converter includes a switching unit for switching two PWM signals inverted with a dead time and outputting the PWM signals, a smoothing circuit for outputting DC power using a waveform output from the switching unit as an input, a variable resistor connected to the switching unit and adjusting a switching time of the waveform output from the switching unit, and a variable resistor controller for sensing a current from an output terminal of the smoothing circuit and setting the resistance of the variable resistor to a resistance corresponding to the sensed current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Korean Patent Application No.10-2004-0107117 filed on Dec. 16, 2004 in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present general inventive concept relates to a DC/DC converter, andmore particularly, to a buck direct current/direct current (DC/DC)voltage converter (hereinafter, referred to as a ‘converter’) to performan improved switching operation by adjusting a variable resistoraccording to an output current of the converter.

2. Description of the Related Art

In general, most electronic devices use a DC voltage (to power an ICsemiconductor) or multiple DC voltage levels produced by an adaptorusing a DC/DC converter to divide a DC voltage of a predetermined level,for example, 5 V, 3.3 V or 8 V from 12 V.

FIG. 1 is a schematic block diagram illustrating a conventional DC/DCconverter 100. The DC/DC converter 100 includes a switching circuit 110,a switching controller 120, a bootstrap circuit 130, and a smoothingcircuit 140.

The switching circuit 110 receives an input power and outputs a squarewave having a predetermined duty ratio by a switching operation.

The switching controller 120 controls the switching operation of theswitching circuit 110. In addition, the bootstrap circuit 130 isassociated with the switching circuit 110 and the switching controller120 and provides power to a circuit for driving the switching circuit110. The smoothing circuit 140 converts the square wave output from theswitching circuit 110 into a DC voltage having a predetermined value asan output power.

FIG. 2 is a detailed circuit diagram illustrating the conventional DC/Dcconverter 100 of FIG. 1.

Referring to FIGS. 1 and 2, the switching circuit 110 includes a pair ofFETs Q1 and Q2. Each input of the FETs Q1 and Q2 has a pulse-widthmodulation (PWM) signal inverted with dead time. The square wave havinga predetermined duty ratio is output from an output node 112 by theswitching operation. A control integral circuit (IC) is used as theswitching controller 120 for controlling the switching operation of theswitching circuit 110 in response to a voltage Vdc and the each input.The switching controller 120 includes a terminal HO for providing aninput signal to the FET Q1, a terminal LO for providing an input signalto the FET Q2, a terminal Vcc for driving the switching controller 120,and terminals Vb and Vs for providing power used to drive the FET Q1 inthe switching controller 120. In this case, a square waveform outputfrom the terminals HO and LO is a PWM signal waveform inverted with deadtime.

The bootstrap circuit 130 includes a bootstrap diode and a capacitor C1.When the output node 112 of the switching circuit 110 is grounded, thatis, when the FET Q1 is turned off and the FET Q2 is turned on, chargesaccumulate in the capacitor C1 via the bootstrap diode from a 5V source.As such, a voltage Vbs is formed between the terminals Vb and Vs. Thevoltage V_(bs) is used to drive the FET Q1. In general, the smoothingcircuit 140 includes passive devices such as a resistor, an inductor,and a capacitor. In FIG. 2, an inductor and a capacitor are used in thesmoothing circuit 140. The smoothing circuit 140 converts the squarewave output from the output node 112 of the switching circuit 110 into aDC waveform to provide a predetermined DC voltage Vout.

A resistor R 122 is connected between the bootstrap circuit 130 and theswitching controller 120sto control the FET Q1 of the switching circuit110 so as to quickly perform the switching operation, which results in ahigh peak voltage at the output node 112 when a state of the FET Q1 ischanged from an off state to an on state, and the high peak voltage mayexceed a radiated EMI margin. FIG. 3 is a diagram illustrating outputwaveforms A and B of the switching circuit 110 of FIGS. 1 and 2 with andwithout a resistor R122, respectively. The waveform A is generated atthe output node 112 when the resistor 122 is not used.

When the resistor R 122 is inserted between the bootstrap circuit 130and the control IC 120. When a gate input impedance of the FET Q1varies, a peak ringing voltage of the output node 112 can be reduced asshown in FIG. 2. The waveform B is generated at the output node 112 whenthe resistor R 122 is used as shown in FIG. 3. However, in this case, aswitching operation time in the switching circuit 110 increases, so thata switching loss is generated.

That is, since the resistor R 122 is set to reduce the peak voltagegenerated at the output node 112 by a maximum load current of theconverter, even when a small load current is generated in the converter,an unnecessary switching loss is generated by an increased switchingtime.

Thus, the peak voltage generated at the output node 112 of the switchingcircuit 110 is reduced, and simultaneously, the switching time of theFET Q1 can be adjusted by a load current of the converter. Thus, amethod to reduce the switching loss is needed.

SUMMARY OF THE INVENTION

The present general inventive concept provides a synchronous buck DC/DCconverter in which a gate impedance of a switching circuit is adjustedaccording to a load current of the converter so that a switching time inan output node of the switching circuit can be adjusted and a switchingloss can be reduced.

Additional aspects and advantages of the present general inventiveconcept will be set forth in part in the description which follows and,in part, will be obvious from the description, or may be learned bypractice of the general inventive concept

The foregoing and/or other aspects of the present general inventiveconcept may be achieved by providing a DC/DC converter including aswitching unit to generate a waveform according to one or more PWMsignals, a smoothing circuit to output a DC power according to thewaveform output from the switching unit, a variable resistor connectedto the switching unit to adjust a switching time of the waveform outputfrom the switching unit, and a variable resistor controller to sense acurrent from an output terminal of the smoothing circuit and to set aresistance of the variable resistor according to the sensed current.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the present generalinventive concept will become apparent and more readily appreciated fromthe following description of the embodiments, taken in conjunction withthe accompanying drawings of which:

FIG. 1 is a schematic block diagram illustrating a conventional DC/DCconverter;

FIG. 2 is a circuit diagram illustrating the conventional DC/DCconverter of FIG. 1;

FIG. 3 is diagrams illustrating output waveforms of a switching circuitwith and without a resistor R of FIG. 2;

FIG. 4 is a block diagram illustrating a DC/DC converter according to anembodiment of the present general inventive concept;

FIG. 5 is a circuit diagram illustrating the DC/DC converter of FIG. 4;

FIGS. 6A and 6B are diagrams illustrating a load current I_(out) and avariable resistor R_(var) of the DC/DC converter of FIG. 4; and

FIGS. 7A through 7C are diagrams illustrating output waveforms of aswitching circuit depending on a change of a variable resistor in theDC/Dc converter of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentgeneral inventive concept, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals refer to the likeelements throughout. The embodiments are described below in order toexplain the present general inventive concept by referring to thefigures.

FIG. 4 is a block diagram illustrating a DC/DC converter 400 accordingto an embodiment of the present general inventive concept.

Referring to FIG. 4, the DC/DC converter 400 includes a switchingcircuit 410, a switching controller 420, a bootstrap circuit 430, asmoothing circuit 440, a variable resistor controller 450, and avariable resistor 460. In this case, the switching circuit 410 and theswitching controller 420 can be referred to as a ‘switching unit’.

The switching circuit 410 includes a pair of FETs and outputs a squarewave having a predetermined duty ratio by a switching operation. Theswitching controller 420 controls the switching operation of theswitching circuit 410. In addition, the bootstrap circuit 430 isconnected to the switching circuit 410 and the switching controller 420and provides power to a circuit to drive at least one of the FETs of theswitching circuit 410. The smoothing circuit 440 converts the squarewave output from the switching circuit 410 into a predetermined DCvoltage.

The variable resistor 460 may be connected between the bootstrap circuit430 and the switching controller 460. The variable resistor 460 mayinclude at least one of passive elements and active elements.

The variable resistor controller 450 senses a load current output fromthe smoothing circuit 440, that is, from an output terminal of the DC/DCconverter 400, and adjusts a resistance of the variable resistor 460 toa predetermined value according to the sensed load current.

FIG. 5 is a circuit diagram illustrating a DC/Dc converter 500 accordingto an embodiment of the present general inventive concept. The DC/DCconverter 500 of FIG. 5 may be similar to the DC/DC converter 400 ofFIG. 4.

Referring to FIG. 5, a switching circuit 510 includes a pair of FETs Q1and Q2. Each input of the FETs Q1 and Q2 has a pulse-width modulation(PWM) signal inverted with dead time. A square wave having apredetermined duty ratio is output from an output node 512 by aswitching operation according to a voltage Vdc and the each input. Acontrol integral circuit (IC) may be used as the switching controller420 to control the switching operation of the switching circuit 510. Theswitching controller 520 includes a terminal HO top provide an inputsignal to the FET Q1, a terminal LO to provide an input signal to theFET Q2, a terminal Vcc to drive the, switching controller 520, andterminals Vb and Vs to provide power to drive the FET Q1 of theswitching controller 520. In this case, a square waveform is output fromthe terminals HO and LO as the PWM signal converted with dead time.

The bootstrap circuit 530 may include a bootstrap diode and a capacitorC1. When the output node 512 of the switching circuit 510 is grounded,that is, when the FET Q1 is turned off and the FET Q2 is turned on,charges accumulate in the capacitor C1 via the bootstrap diode from a 5Vsource. As such, a voltage Vbs is formed between the terminals Vb andVs. The voltage Vbs is used to drive the FET Q1.

The smoothing circuit 540 may include passive devices, such as aresistor, an inductor, and a capacitor. The inductor and the capacitormay be used in the smoothing circuit 540 of FIG. 5. The smoothingcircuit 540 converts the square waveform output from the output node 512of the switching circuit 510 into a DC waveform to provide apredetermined DC voltage.

The above-described structure of the DC/DC converter 500 is differentfrom the structure of the conventional DC/DC converter of FIG. 2in thatinstead of the resistor R 122 having a fixed resistance value, avariable resistor Rvar 560 is connected between the bootstrap circuit530 and the switching controller 520, and the DC/DC converter accordingto the present embodiment is further provided with a variable resistorcontroller 550 to set a variable resistor Rvar 560. In this case, thevariable resistor Rvar 560 may include passive elements, activeelements, or a combination of passive and active elements.

In the present embodiment, the variable resistor controller 550 includesan output sensing unit 552, a variable resistor determining unit 554, astoring unit 556, and a variable resistor adjusting unit 558.

The output sensing unit 552 senses a load current from an outputterminal of the smoothing circuit 540, that is, from an output terminalof the DC/DC converter 500, and transmits the sensed load current to thevariable resistor determining unit 554.

A value of the variable resistor Rvar 560 may be set according to theload current of the converter 500 is stored in the storing unit 556.Here, the load current may be proportional to the resistance of thevariable resistor Rvar 560, as shown in FIG. 6A. In other words, as theload current lout increases, the resistance of the variable resistorRvar 560 increases. It is possible that the resistance of the variableresistor Rvar 560 may be set to predetermined values according topredetermined ranges of the load current. That is, as shown in FIG. 6B,when the load current lout is in a range I₁, the resistance of thevariable resistor Rvar 560 is set to R1, when the load current lout isin a range I₂, the resistance of the variable resistor Rvar 560 is setto R2, and when the load current lout is in a range I₃, the resistanceof the variable resistor Rvar 560 is set to R3.

The variable resistor determining unit 554 extracts a resistance valueof the variable resistor Rvar 560 from the storing unit 556 using theload current sensed by the output sensing unit 552, and transmits theextracted resistance value to the variable resistor adjusting unit 558.The variable resistor determining unit 558 adjusts the resistance of thevariable resistor Rvar 560 according to the extracted resistance value.

Thus, the value of the variable resistor Rvar 560 varies according tothe load current so that a switching time of the switching circuit 510varies.

FIGS. 7A through 7C are diagrams illustrating output waveforms of aswitching circuit depending on a change of a variable resistor in theDC/DC converter 400 and 500 of FIGS. 4 and 5.

Here, I₁<I₂<I₃ and R₁<R₂<R₃.

FIG. 7A shows a waveform generated at the output node 512 of theswitching circuit 510 when the load current is I₁, and the value of thevariable resistor Rvar 560 is set to R₁; FIG. 7B shows a waveformgenerated at the output node 512 of the switching circuit 510 when theload current is I₂, and the value of the variable resistor Rvar 560 isset to R₂; and FIG. 7C shows a waveform generated at the output node 512of the switching circuit 510 when the load current is I₃, and the valueof the variable resistor Rvar 560 is set to R₃.

In other words, even though the load current decreases, the value of thevariable resistor Rvar 560 is adjusted so that a peak voltage generatedat the output node 512 of the switching circuit 510 can be reduced, andsimultaneously, the switching time can be reduced and a switching losscan be accordingly reduced. Although a rising time corresponding to thevalue of the variable resistor Rvar 560 varies according to the loadcurrent, a falling time of the square wave may not be changed, that is,a period of the square wave between a starting point of the rising timeand an end point of the falling time may not be changed but remain same.In addition, a peak voltage of the waveform in the rising time is notchanged.

As described above, in the synchronous buck DC/DC converter according tothe present embodiment, a peak ringing voltage generated by a switchingoperation performed by the switching circuit of the converter is reducedand a switching time is adjusted according to the load current of theconverter so that a switching loss generated in the switching circuitcan be reduced.

Although a few embodiments of the present general inventive concept havebeen shown and described, it will be appreciated by those skilled in theart that changes may be made in these embodiments without departing fromthe principles and spirit of the general inventive concept, the scope ofwhich is defined in the appended claims and their equivalents.

What is claimed is:
 1. A DC/DC converter comprising: a switching unit tooutput a waveform having a first rising time according to a plurality ofPWM signals corresponding to a plurality of power sources; a smoothingcircuit to output a DC power according to the waveform output from theswitching unit; a variable resistor connected to the switching unit toadjust a switching time of the waveform output from the switching unitand to adjust one of the plurality of power sources supplied to theswitching unit; and a variable resistor controller to sense a loadcurrent from an output terminal of the smoothing circuit and to set aresistance of the variable resistor according to the sensed current sothat the switching unit generates an adjusted switching time accordingto the adjusted one of plurality of power sources, wherein the smoothingcircuit outputs another waveform having a second rising time accordingto the set resistance of the variable resistor and according to theadjusted switching time of the waveform output from the switching unit.2. The DC/DC converter of claim 1, wherein the switching unit comprises:a switching circuit having a pair of field effect transistors (FETs) tooutput a square wave having a predetermined period as the waveform by aswitching operation; and a switching controller to control the switchingoperation of the switching circuit.
 3. The DC/DC converter of claim 1,wherein the variable resistor controller comprises: an output sensingunit to sense the load current from the output terminal of the smoothingcircuit; and a variable resistor determining unit to determine apredetermined resistance corresponding to the sensed load current; and avariable resistor adjusting unit to sent set the resistance of thevariable resistor to the determined resistance.
 4. The DC/DC converterof claim 3, wherein the variable resistor determining unit determinesthe resistance to be proportional to the sensed load current.
 5. TheDC/DC converter of claim 3, wherein the variable resistor determiningunit determines the resistance corresponding to a predetermined range ofthe sensed load current.
 6. The DC/DC converter of claim 3, wherein thevariable resistor controlling unit further comprises: a storing unit tostore information about the load current sensed by the output sensingunit and a the resistance corresponding to the load current.
 7. TheDC/DC converter of claim 1, further comprising: a bootstrap circuitconnected to the switching circuit to provide power to drive theswitching circuit.
 8. The DC/DC converter of claim 1, wherein thevariable resistor comprises: at least one of passive elements, activeelements, and a combination of the passive and active elements.
 9. TheDC/DC converter of claim 1, wherein the DC/DC converter comprises asynchronous buck DC/DC converter.
 10. A DC/DC converter comprising: aswitching controller to generate a switching signal according to a firstpower source; a switching circuit to output a waveform according to theswitching signal and a second power source; a smoothing circuit tooutput a DC power according to the waveform output from the switchingcircuit; a variable resistor connected between the switching controllerand the first power source to adjust the first power source supplied tothe switching controller; and a variable resistor controller to sense aload current of the smoothing circuit and to change a resistance of thevariable resistor to generate the adjusted first power source accordingto the sensed load current, so that the switching controller generates amodified switching signal according to the adjusted first power source,the switching circuit outputs a second waveform according to themodified switching signal, and the smoothing circuit outputs a second DCPower according to the second waveform.
 11. The DC/DC converter of claim10, further comprising: a bootstrap circuit -disposed between the firstpower source and the switching controller to supply a voltage to theswitching controller, wherein the switching controller generates theswitching signal according to the voltage.
 12. The DC/DC converter ofclaim 11, wherein the variable resistor is disposed between thebootstrap circuit and the switching controller.
 13. The DC/DC converterof claim 11, wherein the switching circuit comprises an output terminalto output the waveform, and the bootstrap circuit comprises a capacitorconnected between the first power source and the output terminal of theswitching circuit.
 14. The DC/DC converter of claim 10, wherein: theload current comprises a first load current and a second load current;and the waveform comprises a first waveform having a first rising timeand a first peak voltage in the first rising time, and a second waveformhaving a second rising time and a second peak voltage in the secondrising time, in response to the first load current and the second loadcurrent, respectively.
 15. The DC/DC. converter of claim 10, wherein afirst peak voltage and a second peak voltage are same.
 16. The DC/DCconverter of claim 10, wherein the waveform comprises a first waveformand a second waveform having a first period and a second period, thefirst and second periods being identical.
 17. The DC/DC converter ofclaim 10, wherein: the load current comprises a first load current and asecond load current; the resistance of the variable resistor comprises afirst resistance and a second resistance according to the first andsecond load currents, respectively; and the waveform comprises a firstwaveform having a first rising time and a first peak voltage in thefirst rising time, and a second waveform having a second rising time anda second peak voltage in the second rising time, according to the firstresistance and the second resistance, respectively.
 18. A DC/DCconverter comprising: a switching controller to generate a switchingsignal according to a first power source; a switching circuit to outputa first waveform having a first risina time according to the switchingsignal and a second power source; a smoothing circuit to output a DCpower according to the first waveform output from the switching circuit;and a controller to detect a characteristic of the smoothing circuitwith respect to the output DC power and to control the switching circuitaccording to the characteristic of the smoothing circuit to output thefirst waveform having the first rising time and a first peak voltage inthe first rising time, and a second waveform having a second rising timeand a second peak voltage in the second rising time, as the waveform andthe controller to generate a modified switching signal when the firstpower source that supplies the switching controller is adjusted, whereinthe smoothing circuit outputs another DC power according to the secondwaveform and according to the control of the switching circuit by thecontroller.
 19. The DC/DC converter of claim 18, further comprising: abootstrap circuit connected between the first power source and theswitching controller to supply a voltage to the switching controller;and a variable resistor disposed between the bootstrap circuit and theswitching controller to adjust the voltage according to the detectedcharacteristic of the smoothing circuit.
 20. The DC/DC converter ofclaim 18, wherein the characteristic of the smoothing circuit comprisesa load current.
 21. The DC/DC converter of claim 18, wherein a firstpeak voltage and a second peak voltage are substantially same.
 22. ADC/DC converter comprising: a variable resistor; a switching unitcoupled to the variable resistor to output a first waveform having afirst rising time according to (i) a plurality of PWM signalscorresponding to a plurality of power sources and (ii) a firstresistance value provided by the variable resistor; and a smoothingcircuit to output a DC power according to the first waveform output fromthe switching unit, wherein a resistance of the variable resistorcoupled to the switching unit is changeable to adjust a switching timeof the first waveform output from the switching unit and to adjust theat least one power source of the plurality of power sources, wherein theswitching unit outputs a second waveform having a second rising timewhen the resistance of the variable resistor is changed from the firstresistance value to a second resistance value and according to theadjusted switching time of the first waveform output from the switchingunit.
 23. The DC/DC converter of claim 22, wherein the variable resistorcontroller is configured to sense a load current from an output terminalof the smoothing circuit and to set a resistance of the variableresistor according to the sensed current.
 24. The DC/DC converter ofclaim 22, wherein the switching unit comprises: a switching circuithaving a pair of field effect transistors (FETs) to output a square wavehaving a predetermined period as the first waveform or the secondwaveform by a switching operation; and a switching controller to controlthe switching operation of the switching circuit.
 25. The DC/DCconverter of claim 22, wherein the variable resistor controllercomprises: an output sensing unit to sense the load current from theoutput terminal of the smoothing circuit; and a variable resistordetermining unit to determine a predetermined resistance correspondingto the sensed load current; and a variable resistor adjusting unit tosent the resistance of the variable resistor to the determinedresistance.
 26. The DC/DC converter of claim 25, wherein the variableresistor determining unit determines the resistance to be proportionalto the sensed load current.
 27. The DC/DC converter of claim 25, whereinthe variable resistor determining unit determines the resistancecorresponding to a predetermined range of the sensed load current. 28.The DC/DC converter of claim 25, wherein the variable resistorcontrolling unit further comprises: a storing unit to store informationabout the load current sensed by the output sensing unit and theresistance corresponding to the load current.
 29. The DC/DC converter ofclaim 22, further comprising: a bootstrap circuit connected to theswitching circuit to provide power to drive the switching circuit. 30.The DC/DC converter of claim 22, wherein the variable resistorcomprises: at least one of passive elements, active elements, and acombination of the passive and active elements.
 31. The DC/DC converterof claim 22, wherein the DC/DC converter comprises a synchronous buckDC/DC converter.